1. Field of the Invention
The present invention relates to an electrically conductive member, particularly, to an electrically conductive member used in at least one of the transfer roll, the charging roll, the developing roll, the cleaning roll, the transfer belt, the intermediate transfer belt and the intermediate transfer drum arranged around the photosensitive drum (image carrier) included in an electrophotographic printing apparatus such as a copier, a printer, or a facsimile machine.
2. Description of the Related Art
As known in the art, an electrophotographic printing apparatus such as a copier, a printer or a facsimile machine is constructed as shown in FIG. 1. Reference numeral 1 shown in the drawing denotes a photosensitive drum. Arranged around the photosensitive drum 1 are a charging roll 2, a laser beam irradiating section 3, a developing section 4, a primary transfer roll 5 and a cleaning roll 6. An intermediate transfer belt 8 supported by a plurality of support rolls 7a, 7b, 7c extends through the clearance between the photosensitive drum 1 and the primary transfer roll 5. Also, a blade 9 for removing the toner remaining on the intermediate transfer belt 8 is arranged in the vicinity of the intermediate transfer belt 8. Further, a secondary transfer roll 10 is arranged at the position facing the support roll 7b such that a paper sheet 11 is transferred through the clearance between the secondary transfer roll 10 and the support roll 7b. The paper sheet 11 having the toner transferred thereonto is thermally fixed by a fixing device 12, thereby obtaining a printed matter.
The electrophotographic printing apparatus constructed as shown in FIG. 1 is operated as follows. In the first step, an electric field is applied to the charging roll 2 so as to charge the charging roll 2, followed by forming a latent image in the laser beam irradiating section 3 formed on the photosensitive drum 1 and subsequently transferring the toner onto the photosensitive drum 1. Then, a bias is applied to the primary transfer roll 5 so as to transfer the toner attached to the photosensitive drum 1 onto the intermediate transfer belt 8. Further, a bias is applied to the secondary transfer roll 10 so as to transfer the toner attached to the intermediate transfer belt 8 onto the paper sheet 11. Still further, the paper sheet 11 having the toner image transferred thereonto is transferred to the fixing device 12. As a result, the toner image transferred onto the paper sheet 11 is thermally fixed, thereby obtaining desired printed matter.
An electrically conductive member prepared by imparting electric conductivity to an elastic rubber or resin is used in any of the charging roll 2, the primary transfer roll 5, the cleaning roll 6, the intermediate transfer belt 8, and the secondary transfer roll 10. The electrical characteristics required for the electrically conductive members arranged around the photosensitive drum 1 include, for example, the characteristics that nonuniformity of resistance should be small, that the dependency of the resistance on the environment should be low, and that the dependency of the resistance on the voltage should be low. Further, the electrically conductive member is required to exhibit elasticity and, particularly, low hardness and a small compression set. A material prepared by imparting electrical conductivity to an elastic rubber having a high molecular weight is used in general as the electrically conductive member.
The high molecular weight elastic rubber is made electrically conductive by dispersing an ionic conductive agent or an electronic conductive agent into the high molecular weight elastic rubber. However, the ionic conductivity and the electronic conductivity are exactly opposite to each other in their electric characteristics, as shown in Table 1 below. For example, in the case of the electrically conductive member prepared by using an ionic conductive agent, the resistance of the electrically conductive member is lowered under a high-temperature, high-humidity (HH) environment so as to make it impossible to obtain an appropriate current even if the resistance of the electrically conductive member is set at an appropriate value under a normal-temperature, normal-humidity (NN) environment, as shown in FIG. 6. It follows that an image defect is brought about. Also, under a low-temperature, low-humidity (LL) environment, the resistance is increased so as to make it impossible to obtain an appropriate current. An image defect is brought about in this case, too. Incidentally, line a in FIG. 6 denotes the use of an electronic conductive agent, and line b denotes the use of an ionic conductive agent.
TABLE 1DependencyType ofDeterminationCurrentNonuniformityDependencyonResistanceconductivityof resistancespeedin resistanceon voltageenvironmentelevationIonicIonSlowSmallSmallLargeSmallconductivityconcentrationElectronicDistanceFastLargeLargeSmallLargeconductivitybetweenadjacentparticleshavingelectronicconductivity
Also, as shown in FIG. 7, in the case of the electrically conductive member prepared by using an electronic conductive agent, the electrically conductive member is characterized in that the resistance is increased under a low voltage so as to make it difficult to obtain an appropriate current under a low voltage. Incidentally, line a shown in FIG. 7 denotes the use of an electronic conductive agent, and line b denotes the use of an ionic conductive agent. Further, in the case of electronic conductive agent, as shown in FIG. 8, the resistance is varied depending on the mixing amount of the conductive rapidly agent in the region of the intermediate resistance. Still further, as shown in FIG. 9, nonuniformity in resistance is increased among the different parts within one product or among several products due to nonuniform dispersion of the electronic conductive agent in the manufacturing stage. Incidentally, curve a shown in FIG. 9 denotes the volume resistance, and curve b denotes the nonuniformity in resistances. Recently, it has been attempted to moderate the defects of the ionic conductivity and the electronic conductivity by means of hybridization, in which both the ionic conductive agent and the electronic conductive agent are dispersed in, for example, an elastic rubber material. Incidentally, the term “hybridization” denotes complex conductivity including the ionic conductivity and the electronic conductivity.
In general, the hybridization of the ionic conductivity and the electronic conductivity is intended to manufacture an electrically conductive member having a small dependency of the resistance on the voltage and a small dependency of the resistance on the environment. The hybridization is achieved by dispersing an electronic conductive agent such as electrically conductive carbon black or particles of a metal oxide into a rubber compound or resin that has been made conductive in advance by the mixing of an ionic conductive agent, followed by vulcanizing or thermally setting the resin.
However, the electronic conductive agent is unsatisfactory in its dispersion capability and gives rise to a large variation in resistance of the electrically conductive member in the middle resistance region. As a result, it is difficult to achieve subtle control of the resistance relying on the electronic conductivity. Such being the situation, it has been difficult to moderate the defects in the characteristics of the ionic conductivity and the defects in the characteristics of the electronic conductivity by the hybridization.
FIGS. 10A to 10D show the resistance distribution of the press sheets of size 1.5 mm (thickness)×200 mm (width)×300 mm (length), which were prepared by using conductive agents of different conductivity systems. Specifically, shown is the comparison of the nonuniformity in the resistances (the logarithm of resistance: unit of (Ω·cm) depending on the difference in the conductivity system under voltage application of 100V. FIG. 10A covers the case of using an ionic conductive agent. As shown in the drawing, the logarithm of the maximum resistance was 8.05 log(Ω·cm) and the minimum resistance was 7.94 log(Ω·cm). It follows that the difference between the maximum resistance and the minimum resistance was 0.11 log(Ω·cm), supporting that the nonuniformity of resistance was small. FIG. 10B covers the case of using an electronic conductive agent. As shown in the drawing, the maximum resistance was 8.49 log(Ω·cm) and the minimum resistance was 7.81 log(Ω·cm). It follows that the difference between the maximum resistance and the minimum resistance was 0.68 log(Ω·cm), supporting that nonuniformity of the resistance was large. FIG. 10C covers the case of the hybridization performed by the above general method of using both an ionic conductive agent and an electronic conductive agent. As shown in the drawing, the maximum resistance was 7.43 log(Ω·cm) and the minimum resistance was 6.86 log(Ω·cm). It follows that the difference between the maximum resistance and the minimum resistance was 0.57 log(Ω·cm), supporting that the nonuniformity of resistance was not small. Further, FIG. 10D covers the case of the hybridization performed by using an electrically conductive powder according to the present invention. As shown in the drawing, the maximum resistance was 7.88 log(Ω·cm) and the minimum resistance was 7.70 log(Ω·cm). It follows that the difference between the maximum resistance and the minimum resistance was 0.18 log(Ω·cm), supporting that the nonuniformity of resistance was small.
As pointed out above, it is impossible to secure a stable resistance by using an electronic conductive agent or an ionic conductive agent or by the general hybridization using both an ionic conductive agent and an electronic conductive agent, leading to an image defect.
The electrically conductive members are disclosed in patent documents 1 to 5 given below:
Patent Document 1 (Japanese Patent Disclosure (Kokai) No. 4-85341):
Patent document 1 discloses an electrically conductive silicone rubber sponge prepared by mixing an electrically conductive rubber powder obtained from a vulcanized conductive silicone rubber into an unvulcanized silicone rubber. In this case, carbon black is contained as a conductivity imparting agent in the electrically conductive powder.
In general, if carbon black is mixed with an unvulcanized silicone rubber, the vulcanization is retarded, resulting in failure to obtain a uniform sponge. Such being the situation, the technology disclosed in patent document 1 is intended to obtain an electrically conductive silicone rubber sponge having uniform sponge properties by mixing the electrically conductive rubber powder containing carbon black into the unvulcanized silicone rubber. However, patent document 1 does not teach the technical idea of stabilizing the electrical characteristics by the hybridization of the ionic conductivity and the electronic conductivity.
Patent Document 2 (Japanese Patent Disclosure No. 2001-242725):
Patent document 2 discloses an intermediate transfer member comprising a substrate and at least a surface layer of the substrate. It is taught that the intermediate transfer member is characterized in that the surface layer contains both a conductive agent serving to impart electronic conductivity and another conductive agent serving to impart ionic conductivity. Patent document 2 also teaches that carbon black having the fluorination treated surface acts as a conductive agent serving to impart electronic conductivity, and that the conductive agent serving to impart ionic conductivity is selected from the group consisting of a cationic surfactant, an anionic surfactant, an amphoteric surfactant, and a nonionic surfactant.
Patent Document 3 (Japanese Patent Disclosure No. 2002-116638):
Patent document 3 discloses an electrically conductive roller comprising a roller core and an elastic polymer layer formed to cover the roller core. It is taught that the elastic polymer is prepared by dispersing a conductive substance having electronic conductive mechanism into an electrically conductive material to which is imparted a conductive substance having ionic conductive mechanism, and that the elastic polymer has a hardness of 5 to 70° (Asker C hardness).
Each of patent documents 2 and 3 pointed out above refers to the hybridization of the electronic conductivity and the ionic conductivity, which is carried out in general. However, in the method disclosed in each of patent documents 2 and 3, the resistance tends to be varied in the middle resistance region of 6 to 9 log(Ω·cm) as pointed out previously. It should be noted that it is difficult to obtain a stable resistance because of nonuniform dispersion of the electronic conductive agent and the error in the mixing amount of the electronic conductive agent.
Patent Document 4 (Japanese Patent Disclosure No. 2002-229350):
Patent document 4 discloses an electrically conductive transfer roller comprising a first conductive elastic layer formed to cover a core metal, a second conductive elastic layer formed on the first conductive elastic layer, and a coated layer having release property formed on the second conductive elastic layer. According to this patent document, the conductive transfer roller is characterized in that the conductivity by the electronic conductivity, by the ionic conductivity or by the hybrid conductivity of the electronic conductivity and the ionic conductivity is imparted to each of the first conductive elastic layer and the second conductive elastic layer. Concerning the conductive agent, patent document 4 also teaches that it is possible to employ the electronic conductivity produced by electrically conductive carbon, the ionic conductivity produced by, for example, lithium perchlorate, or the hybrid conductivity including both the electronic conductivity and the ionic conductivity.
Patent Document 5 (Japanese Patent Disclosure No. 2002-3651):
Patent document 5 discloses a semiconductive rubber composition of an island-ocean structure comprising a polymer consecutive phase consisting of a rubber material having ionic conductivity and a polymer grain phase consisting of a rubber material having electronic conductivity. It is taught that the rubber material having the ionic conductivity contains mainly a raw material rubber A having a volume resistivity not higher than 1×1012Ω·cm. It is also taught that electrically conductive particles are mixed into a raw material rubber B so as to make conductive the rubber material having the electronic conductivity.
Patent document 5 also teaches that a master batch is prepared by adding electrically conductive particles such as electrically conductive carbon black to the raw material rubber B alone, followed by blending the resultant master batch with the raw material rubber A so as to prepare a semiconductive rubber composition. In addition, patent document 5 teaches that the raw material rubber A is a polar rubber, and that the raw material rubber B is incompatible with the raw material rubber A. What should be noted is that the raw material rubber B forming the polymer grain phase in blended with the raw material rubber A forming a polymer consecutive phase in the unvulcanized state, followed by vulcanizing the mixture of the raw material rubbers A and B. It follows that the island-ocean structure of the polymer consecutive phase and the grain phase is formed by utilizing the incompatibility between the raw material rubbers A and B.
In the semiconductive rubber composition disclosed in patent document 5, however, it is necessary for the raw material rubbers A and B to differ from each other in polarity and to be incompatible with each other in order to form the island-ocean structure. Also, in order to form the island-ocean structure, it is necessary for the Sp value of the raw material rubber B to be smaller than that of the raw material rubber A, and it is necessary for the difference in the Sp value to be large. It follows that the range of selection of the polymers used is limited. It should also be noted that, since the island-ocean structure is formed by applying vulcanization to the raw material rubber after blending of the unvulcanized raw material rubber, the process conditions such as the viscosity and the blending ratio of the raw material rubbers A and B as well as the temperature and the time in the mixing process are much restricted in order to obtain a prescribed island-ocean structure. Further, patent document 5 teaches the phenomenon that some electrically conductive particles are allowed to migrate into the polymer consecutive layer, indicating that the semiconductive rubber composition disclosed in this patent document is insufficient for the control and stabilization of the electrical characteristics.